1. Field of the Invention
The present invention relates an outboard motor especially having an improved structure of an intake system and a fuel supply system for the outboard motor.
2. Description of the Related Art
A certain type of an engine, i.e., internal combustion engine, utilizes a carburetor as a device for supplying a fuel-air mixture into the internal combustion engine. The carburetor is designed to achieve the maximum performance through a combination of several jets in order to meet the requirements in performance and operation of the engine. Such a carburetor is therefore inappropriate for adaptation to variation of driving conditions of the engine and surroundings, according to circumstances.
Therefore, there have recently been widely used internal combustion engines provided with a fuel injection device in place of the carburetor. The fuel injection device directly injects fuel through a fuel injector into an intake passage of the engine in an amount. This amount is judged as the most appropriate amount required currently, based on a correction value therefor, which is determined by obtaining internal and external information on the engine with the use of various sensors, and then processing the thus obtained information by a computer. Such a fuel injection device can contribute an improvement of a combustion efficiency and capacity of the engine and can achieve a reduced fuel-consumption due to the injection of the fuel in the minimum amount as required, thus providing many advantageous effects.
With respect to an example of arrangement of the fuel injectors in the engine for an outboard motor, which is provided with the fuel injection device, Japanese Laid-Open Patent Application No. HEI 8-310487 discloses the fuel injectors, which are mounted on a connection of an intake manifold to a cylinder head of the engine so as to inject fuel to an intake port formed in the cylinder head.
The engine for an outboard motor is surrounded with a cowl, and an intake operation is carried out within the cowl so as not to suck water during the intake operation.
According to the above-described setting position of the fuel injectors and the injection direction of fuel, there is a short distance between the fuel injector and the combustion chamber formed in the cylinder head, and it takes merely a short period of time for the injected fuel to reach the combustion chamber, with the result that the fuel is sucked into the combustion chambers in a completely atomized state (i.e., a fully mixed state).
As a result, a HC (hydrocarbon) content in an exhaust gas increases, and the fuel, which has still been kept in a liquid state, passes through a gap around a piston ring to enter a crankcase, thus leading to declination in concentration of engine oil and being inconvenient or disadvantageous.
Especially, the internal combustion engine for an outboard motor has a cooling system in which cooling water is taken from the outside the engine to cool the same, thus leading to a relatively low temperature of the cooling water. In addition, a low speed driving operation such as a trawling operation or an idling operation of the engine is carried out with high frequency, thus leading to a relatively low temperature of the engine itself. These characteristics of the internal combustion engine for an outboard motor cause an easy occurrence of the inconveniences or disadvantages mentioned above.
On the other hand, the temperature of air existing in the cowl, with which the internal combustion engine is surrounded, increases by heat of the engine, and the thus heated air is taken into the engine. In these circumstances, increase in temperature of the air may lead to reduction of intake efficiency and deterioration of engine capacity.